As semiconductor devices have shrunk in size, deep ultraviolet (DUV) lithography has come into use for the manufacture of sub-0.35 micron line geometry semiconductor devices.
One problem with DUV lithography, as well as with conventional i-line lithography, is that the high reflectivity of polysilicon, metal, and poly/metal stacks have made small feature patterning and critical dimension (CD) control of photolithographic resist very difficult.
To solve this problem, different anti-reflective coatings (ARCs) have been developed which work by phase shift cancellation of specific wavelengths to provide uniform resist patterning. The ARCs are specifically designed so that the light reflected from the resist/ARC interface is equal in amplitude but opposite in phase to the light reflected from the ARC/reflective layer interface.
It has been found that there are certain line width variations which are due to the ARC not being able to reduce the reflective layer reflectivity to a minimum. The reflectivity causes problems with the resist which have been corrected in part by the use of bottom anti-reflective coatings (BARCs) underneath the resists. However, in some applications the BARC serves two purposes and must be of a certain thickness to serve as the hard mask for poly or metal etch while at the same time it must be of a predetermined thinness to minimize reflective layer reflectivity. In the past, the compromise thickness has been one which reduces, but does not minimize, reflectivity.
Silicon oxynitride by itself has been found to be a good BARC material. However, the thickness of silicon oxynitride required to perform the function of a good hard mask is too thick to minimize reflectivity.
In essence, the silicon oxynitride BARC serves two functions during semiconductor memory manufacturing: (1) as a hard mask during self-aligned etch (SAE) and during self-aligned-source etch; and (2) as a bottom anti-reflective layer for photolithography at second gate masking. In order for the silicon oxynitride to act as an adequate hard mask, it must be approximately 100 nm thick in current applications. However, the ideal BARC thickness is approximately 30 nm, which is not thick enough to survive the SAE and self-aligned source etching processes.
Thus, a 100 nm silicon oxynitride BARC is currently required and this causes line widths to be non-uniform. The non-uniform line width is a result of lensing reflections of light into the resist from undulations in the topography of reflective layers under the BARC that are not completely phase cancelled by the BARC.
A number of different solutions have been tried. None of these has been successful in producing the uniform line widths. Further, as the critical dimensions are reduced, the problem becomes more severe.